Presently, researches are underway regarding a write-once-read-many optical recording medium which permits recording at wavelengths of a blue laser or shorter.
Regarding the existing write-once-read-many optical recording medium, information pits are formed by applying a laser beam to a recording layer comprising an organic dye to cause an alteration in the refractive index mainly due to the decomposition or alteration of the organic material. Therefore, the optical constants and the decomposition behavior of the organic dye used for the recording layer are important factors to form preferable recording pits.
Hence, in order to achieve a write-once-read-many optical recording medium compliant with a blue laser beam, it is necessary to select a material that has appropriate optical properties and decomposition behavior at blue laser wavelengths as an organic material used for the recording layer.
However, no organic materials that comprises a high-to-low (i.e. the reflectivity at a recording mark is lower than that at a non-recording portion) recording polarity in the region of blue laser wavelengths and is able to offer favorable optical properties at blue laser wavelengths have been put into practical use.
The molecular skeleton must be downsized or the conjugate system must be shortened in order to produce an organic material having absorption band in the vicinity of blue laser wavelengths in order to obtain an organic recording medium having absorption band in the vicinity of blue laser wavelengths. However, this decreases absorption coefficient and consequently decreases the refractive index, which is followed by decreased reflectivity during non-recording period and smaller modulation.
In other words, although there exist many organic materials having an absorption band in the vicinity of blue laser wavelengths, and it is possible to control the absorption coefficients, but these materials do not have sufficiently high refractive indices. Therefore, it is difficult (but not impossible) to realize the high-to-low polarity with an organic material.
Given above, the recent trend is to make the recording polarity low-to-high in order to use an organic material for a write-once-read-many optical recording medium compliant with a blue laser beam.
However, it is undeniable that the recording polarity is preferably high-to-low since, in terms of recording apparatus, adoption of low-to-high recording polarity loses the compatibility with a read-only optical recording medium (ROM) and other conventional optical recording media.
On the other hand, a technology that adopts a recording layer with an inorganic material has been proposed, for example in Patent Literatures 1 to 2, as a write-once-read-many optical recording medium that is compliant with a blue laser beam whose recording polarity is high-to-low in Also, the usability of a recording layer comprising an oxide of a metal or a semi-metal, especially an oxide of bismuth, as a main component is proposed in Nonpatent Literatures 1 to 2.
In addition, similarly to these conventional technologies, the following technologies have been disclosed regarding materials comprising bismuth or an oxide of bismuth.
For example, Patent Literature 3 proposes a technology regarding an amorphous ferromagnetic oxide with a general formula Ax(MmOn)y(Fe2-O3)z, where A as an oxide, M as an element and x, y and z as fractions are defined. Also, Patent Literature 4 proposes a metal oxide comprising 50% or more of an amorphous phase with an oxide MmOn and predefined fractions x, y and z, with respect to the general formula Ax(MmOn)y(Fe2O3)z, and a manufacturing method thereof. Patent Literature 5 proposes the range of composition x regarding an amorphous compound comprising a formula of (B2O3)x(Bi2O3)1-x and a quenching method thereof. In addition, Patent Literature 6 discloses a technology relating to a bismuth-iron amorphous compound material comprising a composition of (Bi2O3)1-x(Fe2O3)x (where 0.90≧x>0). These are all related to a transparent, ferromagnetic amorphous oxide material, which is applied for a magnetooptical recording medium, a function element that magnetically controls a light, a magnetooptical sensor, a transparent conductive film and a piezoelectric film. Furthermore, the conventional technologies described in Patent Literatures 3 to 6 are anchored by patents relating to materials and manufacturing methods, and these literatures do not mention an applicability to a write-once-read-many optical recording medium.    Patent Literature 1 Japanese Patent Application Laid-Open (JP-A) No. 2003-200663    Patent Literature 2 JP-A No. 2003-203383    Patent Literature 3 JP-A No. S61-101450    Patent Literature 4 JP-A No. S61-101448    Patent Literature 5 JP-A No. S59-8618    Patent Literature 6 JP-A No. S59-73438    Nonpatent Literature 1 Write-Once Disk with BiFeO Thin Films for Multilevel Optical Recording, JJAP, Vol. 43, No. 7B, 2004, pp. 4972    Nonpatent Literature 2 Write-Once Disk with BiFeO Thin Films for Multilevel Optical Recording, JJAP, Vol. 44, No. 5B, 2005, pp. 3643-3644